64 THE GEOLOGY OP MINNESOTA. 



[The Beaver Bay diabase. 



group. This diabase has an intimate connection with the red rock, and was appar- 

 ently cotemporary with the gabbro and the red rock. It is believed to be due to the 

 first (and the greatest) flow-movement from the gabbro mass toward the Lake 

 Superior basin. If there were lavas that preceded it they were unimportant, and of 

 the nature of volcanic ejections, superficial, local and easily removed or reincorpo- 

 rated when the general motion began. It contains many anorthosyte or feldspar 

 masses, and a few of red rock. It is the matrix which encloses the boulders forming 

 a great pudding-stone seen near Beaver bay at different points. Petrographically it 

 is usually a diabase, in that its structure is ophitic, but this is not always the case. 

 Its thickness is sometimes several hundred feet. In many places it has been called 

 gabbro, as at Short Line park, near Duluth, where it not only shows the banded 

 structure seen sometimes in the gabbro, and locally carries a notable amount of iron 

 ore, but is also seen to be vesicular, like a surface flow.* It has an irregular and 

 sometimes a gradational contact on the red rock, quite similar to that of the gabbro 

 on the 'red rock. Such irregularities are common in the country about South Brule 

 lake and eastward. It was molten at about the same time as the red rock, as shown 

 by an inspection of the relations of these two rocks about the shore of Beaver bay, 

 yet it is cut by the red rock, and is the bearer of detached pieces of the same. The 

 rock of the Great Palisades lies upon it, but it forms the crest of the Sawteeth 

 mountains. 



In moving from the area of the gabbro mass this sheet of diabase first encoun- 

 tered the Animikie, which is now represented by the red rock belt. If this rock 

 occupied the prominence which it does at the present time, the diabase, once having 

 surrounded it, would have flowed rapidly down the southern decline, leaving a com- 

 paratively thin covering over the red rock obstruction. If the movement continued 

 long, as is probable, the action of the red rock was to form a kind of reef over which 

 the diabase must have passed more quickly and in an attenuated volume, but further 

 south, gathering in greater amount, was not only slower to solidify, but on complete 

 solidification assumed more nearly the petrographic characters of the parent mass. 

 Therefore, ascending the streams that enter lake Superior, such as Poplar river, or 

 Temperance river, the observer comes upon coarse-grained conditions of this rock 

 which are not distinguishable from the real gabbro, and such phases also appear at the 

 shore line of lake Superior in the vicinity of Beaver bay, as well as at and near 

 Duluth. During the lapse of the ages since this flow took place, the thin surface 

 remnant which was left on the red rock belt has been destroyed and wholly removed, 

 thus isolating the Beaver Bay diabase from its parent mass and constituting it the 

 "great basal flow" of the Keweenawan defined by Irving. 



*By Mr. Elftman this has been included in the later surface lavas of the Manitou series. American Geologist, vol. xxii, plate 

 VII, September, 1898. 



